Most digital circuits use oscillators to provide clocking for synchronous digital devices. A wide range of oscillators, for different applications, are available that provide highly accurate and frequency-selective signals. One category of oscillators is crystal oscillators, which employ vibrating crystals to create electrical signals with very precise frequencies. These oscillators typically provide a stable clock signal to stabilize frequencies for radio transmitters/receivers and for digital integrated circuits.
Crystal oscillators commonly include an amplifier and a crystal in the amplifier's feedback loop. In certain applications, such as mobile phones, the crystal oscillator is frequently turned off when not in use, to save power. In these applications, it is essential that the crystal oscillator achieve stable oscillations quickly, every time it is powered back on.
A few techniques can be employed to reduce the start-up time of crystal oscillators (i.e., the time taken to attain sustained stable oscillations after the power is turned on). Some techniques attempt to reach the bias point of the amplifier quickly. One such method shorts the crystal during start-up, thereby allowing the amplifier to reach its bias point faster. Reaching the bias point faster ensures that oscillations start quickly but does not speed-up the stabilization of oscillations in any manner.